1. Field of the Invention
The present invention relates to a toner for electrostatic-charged image developer used for developing electrostatic latent images formed by an electrophotographic process, electrostatic recording process, etc., the production method of the toner, an electrostatic-charged image developer, and an image-forming process.
2. Background Art
A process of visualizing an image information through electrostatic latent images, such as an electrophotographic process, etc., are utilized in various fields at present. In an electrophotographic process, an electrostatic-charged image (a latent electrostatic image) is formed on a photoreceptor by electrostatic charging and light-exposure steps, the electrostatic latent image is developed by a developer containing a toner and visualized through a transfer step and a fixing step.
As the developer used in this case, a two-component developer composed of a toner and a carrier and a one-component developer using a magnetic toner or a non-magnetic toner singly are known.
For producing these toners, a knead-grinding production method of melt-kneading a thermoplastic resin together with a pigment, an electrostatic charge controlling agent, and a surface lubricant such as a wax, etc., and after cooling, finely grinding and classifying the kneaded mixture is usually used. If necessary, inorganic or organic fine particles are sometimes added to the surfaces of the toner particles for the purpose of improving the fluidity of the developer and the cleaning property. These methods can produce a considerably excellent toner but have the following problems.
That is, in an ordinary knead-grinding production method, the form of the toner and the surface structure of the toner formed are irregular, they are finely changed by the grinding property of the using material and the condition of the grinding step, and it is difficult to positively control the form and the surface structure of the toner formed. Also, in the knead-grinding method, there is a restriction in the range of selecting the material. Practically, the dispersed product of a resin and a colorant is sufficiently brittle and must be finely ground by a production apparatus capable of being used economically.
However, when the dispersed product of a resin and a colorant is brittle, it sometime happen that a fine powder is generated and also the form of the toner is changed by a mechanical shear in a developing apparatus, etc. As the result thereof, in a two-component developer, the fine powder is fixed to the surface of the carrier to accelerate the deterioration of the charging property of the developer, and in a one-component developer, the particle size distribution is enlarged to cause scattering of the toner and, by lowering of the developing property caused by the change of the toner form, the deterioration of the image quality is liable to occur.
Also, in the toner to which a large amount of a lubricant is internally added, according to the combination of a thermal plastic resin, exposing of the lubricant onto the surface of toner is frequently influenced. Particularly, in the combination of a resin which is a little hard to be ground because of the increase in the elasticity by a high molecule weight component and a brittle wax such as polyethylene, many polyethylene is exposed on the surface of the toner. This is advantageous for the lubricating property at fixing and cleaning of the untransferred toner, but polyethylene on the surface layer is easily shifted by a mechanical force to liable to stain a developing roll, a photoreceptor, and a carrier, which results in lowering the reliability of the developer.
Furthermore, when the form of the toner becomes irregular, even by adding a fluidity aid, the fluidity of the toner cannot be sufficiently ensured, and during using the developer, fine particles on the toner surface transfer to the concave portions of the toner by a mechanical shearing force to lower the fluidity of the toner with time, and also the fluidity aid causes embedment in the inside of the toner to reduce the developing property, the transferring property, and the cleaning property of the toner. Also, when the toner recovered by cleaning is returned into the developing apparatus and reused, the image quality is liable to further lower. To prevent the occurrence of these problems, when the amount of the fluidity aid is more increased, the problems that black points form on the photoreceptor and the particles of the fluidity aid are scattered, etc., occur.
Recently, as a method of positively controlling the form and the surface construction of a toner, a production method of a toner by an emulsion polymerization aggregation method is proposed in Japanese Patent Laid-Open Nos. S63-282750 and H06-250439. In these methods, a dispersion of resin fine particles are generally prepared by an emulsion polymerization, on the other hand, a dispersion of a colorant formed by dispersing a colorant in solvent is prepared, both dispersions are mixed to form aggregated particles corresponding to the toner particle sizes, and by heating them, they are melted and coalesced to product a toner. According to the method, the toner form can be controlled to some extent and the charging property and the durability can be improved, but it is difficult to control the particle sizes of the lubricant and the colorant in the toner and the positions of the particles. As a result, there sometimes leave problems in the releasing property of a fixing sheet at fixing and the transparency of an OHP output image.
As described above, in the electrophotographic process, in order that a toner stably keeps the characteristics even under various mechanical stresses, it is necessary that by restraining the exposure of the lubricant onto the toner surface and by increasing the surface hardness without reducing the fixing property, the mechanical strength of the toner itself is improved and the sufficient charging property and the fixing property are compatible with each other.
Recently, the requirement of obtaining a high-quality image is increased and particular, in a color image formation, the tendency of small sizing becomes remarkable for realizing highly precise images. However, in a method of simple small sizing of a toner with the particle size distribution in prior art, by the existence of the toner of a fine powder side, the problems of staining the carrier and the photoreceptor and scattering of the toner become severe and it is difficult to simultaneously realize a high-quality image and a high reliability. For solving the problems, it becomes important to form a sharp particle distribution and make small-sizing of the particle sizes.
Also, in a digital full color copying machine or printer, after color-separating a color image original by each filer of B (blue), R (red) and G (green), a latent electrostatic image made of dots having sizes of from 20 to 70 .mu.m corresponding to the original is developed by utilizing a subtractive process using each developer of Y (yellow), M (magenta), C (cyan), and Bk (black). Because, as compared with a black and white copying machine, in a digital full color copying machine, etc., it is necessary to transfer large amounts of developers and also it is necessary to correspond to dots having a small size, the uniform charging property, the durability, the toner strength, and the sharpness of the particle size distribution become more and more important. Also, for meeting the high copying speed of the copying machines, etc., and energy saving, etc., a lower temperature fixing property becomes necessary. By considering these points, the toner produced by an aggregation.melt coalescent method suitable for producing small-sized particles having a sharp particle size distribution has excellent characteristics.
In a full color copying or recording machine, it is necessary to sufficiently mix color toners and in this case, the improvement of the color reproducibility and the transparency of OHP images become indispensable.
In general, as a lubricant component, a polyolefin-based wax is added in the toner by internal addition for the purpose of preventing the occurrence of a low-temperature offset at fixing. Also, together with this, a very small amount of a silicone oil is uniformly coated on fixing rollers to improve a high-temperature offset property. Thus, there are problems that the silicone oil adhere to a transfer material to cause a sticky unpleasant feeling, etc.
Thus, in Japanese Patent Laid-Open No. H05-61239, a toner for oilless fixing of incorporating a large amount of a lubricant component in the toner without coating an oil on fixing rolls is provided. However, when a large amount of a lubricant component is added into the toner, the releasing property can be improved to some extent, but a binder resin component becomes compatible with the lubricant and oozing of the lubricant becomes ununiform, whereby the stability of releasing is hard to obtain. Also, the liberated component of the lubricant sometimes causes the hindrance of electrostatic charging.
Also, the dispersibility of a colorant in a color toner causes an interaction with the lubricant to form the aggregate of the colorant, which causes the problems of reducing the transparency of OHP images, the hindrance of coloring, etc.
Thus, for solving these problems, in Japanese Patent Laid-Open No. H02-105163, it is proposed to improve the involving property and the oozing property of the lubricant by positively introducing a resin having a polar group. However, in the method, the oozing property of the lubricant can be improved to some extent and the involving property thereof can be improved, but the method scarcely has the effects of controlling the position of the lubricant in the toner and improving the dispersibility of a pigment in the toner, and the overall fixing properties such as the releasing property, the sticking property of the fixed image, the transparency of OHP images, the bending strength of the fixed image, etc., cannot be improved. Also, the insufficient dispersing property of the colorant, etc., gives large influences on the electrostatic charging faculty.
Also, in Japanese Patent Laid-Open No. H04-188156, it is proposed to use a colorant previously subjected to a surface treatment with a monomer component of a binder resin, a wax component, etc., for improving the dispersibility of the colorant. In the method, the dispersibility of the colorant is improved to some extent but the wax holds the colorant particles therein and thus the colorant particles are aggregated with each other without controlling the dispersibility of the pigment in the inside of the toner, whereby it is difficult to simultaneously satisfy the releasing stability of the fixing sheet and the transparency of OHP images.
Also, to improve the aggregating force of a binder resin for the purpose of improving the releasing property of a fixing sheet, a method of adding a crosslinking agent component to the binder resin is proposed in Japanese Patent Laid-Open Nos. S59-218459 and S59-218460. However, when a crosslinking agent component is simply added to a binder resin, the aggregating force of the binder resin is improved and the releasing property is also improved, but because the rigidity of the binder resin itself is increased, the bending resistance of the fixed image becomes poor. The reduction of the bending resistance of a fixed image causes an image defect at bending the fixed image on paper and greatly lowers the image quality of the output images, the bending strength is an important property for practical use. Furthermore, when the melt viscosity of a binder resin is increased, the smoothness of the surface of the fixed image is reduced, the gloss of the fixed image and the transparency of OHP images are lowered and, in a full color image, a sufficient color mixing property becomes hard to obtain.
Furthermore, in Japanese Patent Laid-Open Nos. H04-69666 and H09-258481, a method of improving the apparent aggregating force of a binder resin by adding a high molecular weight component to the binder resin is proposed.
In these methods, the flexibility of the fixed image itself is improved to some extent but it is difficult to control the crosslinking (entangling) density of the binder resin and the crosslinking molecular weight and it is difficult to simultaneously obtain stably the releasing property and the transparency of OHP images while maintaining the bending resistance of the fixed image.